You have gotten good advice here, and I can only second the points that have been made.
I personally prefer WD drives, but that’s purely because I have had a few failures with Seagate external drives. That’s not based on real science as the sample size is of course way too small.
For a desktop, I would strongly argue in for the much less expensive and higher capacity spinning hard drives for backups. Like others, I have two bus-powered drives velcro’d to the under side of my desk, one for TimeMachine and one for a CCC bootable clone. For backups of this sort, SSDs are in my opinion an unnecessary expense, especially if you are backing up a desktop where the time for the backup to take place is longer. If you are backing up a laptop that will only be able to be connected to the drive for a short period of time (as per @JohnAtl), then an SSD may be worth the added cost, especially as your internal drive is probably smaller and hence a smaller external SSD would suffice. I favor drives at least 2x the size of the internal drive for both TM and CCC, although I actually use a much larger one for CCC since these bus-powered drives can be purchased so cheaply on sale, and I don’t delete the “safety net” backups.
Externally powered drives are, I think, helpful if you need a faster drive rotational speed as some are 7200 RPM drives while the bus powered ones are pretty much all 5400 RPM, and again I don’t think the faster rotational speed is needed for backups.
It is a reasonable point that have an SSD based clone is useful if you ever have to run your computer booted off the clone, but that to me is a relatively unlikely scenario for most of us. If your internal drive died on laptop or desktop you would like be taking it in for repair/replacement and hence the clone wouldn’t be useful anyway, When the repair machine is to be restored, yes the clone process going back will take a bit longer, but it’s a one time event.
However, if you are, for instance, frequently traveling with your laptop and need to be assured that if your drive were to die while traveling you could continue to work with the speed of an SSD, that might be a use-case for an external SSD clone. For instance, when I give talks, I will often bring a bootable clone of my laptop so that if my drive dies in transit I will not be stuck. (I also bring a spare copy of the talk on a USB stick and I could also grab a copy from BackBlaze or from my home machine via ResilioSync, but since the rest of the world is so Window-centric I might not be able to find a spare Mac to load my Keynote file). Thankfully I have never had to run out to buy an emergency Mac to give a talk!
Long winded, but hopefully helpful.